1. Field of the Invention
The present invention relates to novel multitubular sheaths particularly useful for electric storage batteries and to novel methods for making such sheaths.
2. Description of the Prior Art
While for convenience the invention will be described with reference to multitubular sheaths for electric storage batteries and is particularly suited thereto, it is not necessarily limited to such applications and could for example be used in the production of multitubular fabric filters for liquids or gases.
It has been proposed in British Pat. No. 1,053,747 to make single tube sheaths for electric storage batteries by impregnating a single porous glass fibre tube in flattened form with a thermosettable resin, namely a solution of phenolformaldehyde resin in ethanol, drawing the wet tube onto a rod having an entry end in the form of a shepherds crook, that is having a curved end extending around about 180.degree., and then extending for a short length out away from the shaft at 90.degree. thereto.
The flattened tube is thus opened out to tubular form and in this form is heated to cure and set the liquid resin. It is then drawn off the rods and cooled and cut to shape.
We have attempted to apply this process to the production of multitubular sheaths on a continuous basis. However, we have found that this cannot be done satisfactorily.
Thus when a sheath having more than one tube is being formed, and especially one having five or more tubes, the fabric has to bunch together in order to go onto the ends of the rods. This results in frequent jamming with the shepherds crook form of rods and build up of resin deposits on the rods also increases the frequency with which the fabric jams on the rods.
We have found however that by varying the shapes of the rods and also by changing the resin and the heating and cooling sequence, these problems can be substantially reduced.
The invention moreover extends in its broadest aspects to a novel multitubular sheath. The term multitubular is defined for the purposes of this specification as meaning a sheath having at least five tubes arranged side by side.
Thus according to this aspect of the invention, a multitubular sheath comprises a porous fabric defining tubes lying parallel to each other and desireably spaced by a web of fabric having a width less than the diameter of the tubes and preferably less than 20% of the diameter of the tubes, impregnated with a polymer composition such that the impregnated material has an air permeability of 0.5 to 20 liters of air/min./sq. cm./cm. head of water, the melting point of the thermoplastic polymer being at least 20.degree. C. below the temperature at which the material of the fabric melts or chars or the composite material being deformable at a temperature, the deformation temperature, at least 20.degree. C. below the temperature at which the material of the fabric melts or chars.
In a preferred form of the invention a multitubular sheath comprises a porous fabric defining tubes lying parallel to each other, the fabric being composed of at least 50% and desirably 80% to 100% of heat shrinkable fibres, which either have been preshrunk or are shrunk in the fabric, the fabric being impregnated with polyacrylic polymer composition, the fabric being rendered substantially entirely resistant to shrinkage prior to introduction onto the rods.
The polyacrylic polymer composition is preferably a true thermoplastic material and preferably comprises a polymethylmethacrylate. The polyacrylic material, when cast as a thin void free film by evaporation at a temperature above 40.degree. C., preferably has a tensile strength on the tensile test of DIN 53455 of 30 to 40 e.g. 35 N/mm.sup.2 at a pulling speed of 100 mm per minute, and an elongation at break .epsilon.R of 2% to 6%, preferably about 4%.
The material on the torsion pendulum test of DIN 53455 preferably has a T.DELTA. maximum at a frequency of 1 hour of about +55.degree. C. to +60.degree. C. e.g. +58.degree. C.
The material is preferably further characterized by having a density of 1.0 to 1.1, e.g. 1.06 grams/cc. Desirably it has an actual viscosity at 20% (Brookfield viscometer#6) of 500 to 4000 mPAS.
The average particle size as a 50% aqueous suspension is preferably 0.15 micromicrons. The suspension preferably has a PH of 7.+-. 0.2 and the minimum film forming temperature is preferably about 40.degree. C.
The fabric is preferably made entirely of polyester fibres and is preferably non woven and may be made of 1 to 4 or 2 to 3 inch long fibres having a denier of 1 to 5, e.g. 2 to 4. The fabric before impregnation may have a heat shrinkability in excess of 10% especially 11% to 20% or 40%.
The heat shrinkability of a fabric is defined herein as the average of the % shrinkage of a sample along the machine length and across the machine length of the fabric when a flat square sample is rested on a flat surface and brought to a temperature of 195.degree. C.
The fabric is preferably made from synthetic organic polymer materials and is preferably a polyester or polyamide material. It may be woven, knitted or non woven. The web between adjacent tubes, when present, may be made of two layers of fabric as when the tubes are made by attaching two layers of fabric to each other along spaced apart lines preferably by stitching or may be a single layer as when the web is produced as an integral stage of knitting or weaving the fabric. In one form of the invention the polymer is preferably a thermoplastic polymer highly resistant to attack by battery environments especially lead acid environments. Preferred polymers are thermoplastic phenol formaldehyde resins (NOVOLAKS), or polyesters or polyacrylic resins, e.g. polymethylmethacrylates.
The polymer compositions used are desirably also elastomeric, namely having elongations at break as void free samples of at least 20% so that the impregnated polymer in the fabric maintains its integrity during the processes used to form the tubes and can accommodate changes in active material volume in the battery during use.
Thus, while plasticized rigid polymers can be used, polymers which have an intrinsic degree of elasticitiy are preferred.
In another form of the invention, the web is selected to be made of thermoplastic fibres and the impregnating resin is a thermosetting resin which is used in an amount such that the impregnated web or composite material is still heat deformable.
The sheaths preferably contain 15% to 35% by weight of impregnated polymer based on the dry weight of the unimpregnated fabric per unit area.
The invention also extends to methods of making such multitubular sheaths.
Thus according to this aspect of the invention, a method of making a multitubular sheath comprises providing a porous multitubular fabric in collapsed form impregnated with 15 to 35%, e.g. 5 to 20% by weight of polymer, the polymer of the composite either being a thermoplastic polymer having a melting point at least 20.degree. C. below the temperature at which the material of the fabric melts or chars, or the composite material being deformable at a temperature, the deformation temperature, at least 20.degree. C. below the temperature at which the material of the fabric melts or chars, and continuously feeding each tube simultaneously onto an array of parallel metal rods having straight sizing portions while holding the rod in a fixed position, the fabric at least immediately prior to the sizing portion being substantially at ambient temperature, and heating the fabric on the sizing portions so as to permit the fabric to conform to the rod and thereafter cooling the fabric on the rods to a temperature such as to permit the thermoplastic resin, if used, to solidify or the deformable composite material to set to shape and the tubes to become self supporting and feeding the fabric off the ends of the rods and cutting it to the required length.
When a thermoplastic polymer is used the heating is preferably carried out at a temperature in excess of the melting point of the thermoplastic polymer but below the melting or charring temperature of the material for a time sufficient to melt the thermoplastic polymer.
The cooling is preferably carried out at a temperature at least 10.degree. C. below the melting point of the thermoplastic resin, or the deformation temperature of the deformable composite material, thus an ambient air stream (15.degree. C.) is effective for a resin melting at 80.degree. C.
In a preferred form of the invention we assist the continuous feeding of the fabric onto the rods by locating a floating mandrel in each tube, this being a short rod, i.e. shorter than the main rod and preferably about 10% to 50% of the length of the main rod, which abuts against the inlet end of the main rod.
The continuous feeding of the fabric onto the rods can also be assisted by heating the fabric before it passes onto the floating mandrels and then allowing or causing the fabric to cool as it passes over the floating mandrels and onto the rods. The temperature of the fabric thus drops to ambient temperature or thereabouts before it reaches the inlet end of the main rod.
The floating mandrel preferably has a diameter or a cross section less than that of the sizing portion of the rod, preferably in the range 80% to 95% of that of the main rod.
The cross section of the rod need not necessarily be circular though this is preferred.
The invention also extends to apparatus for carrying out the aforementioned method.
Thus according to this aspect of the invention, an apparatus for making multitubular sheaths comprises an array of rods, each having a sizing portion of a diameter or cross section corresponding to the internal diameter or cross section desired for the tubes, and an inlet portion of smaller diameter or cross section than the sizing portion, but having a cross sectional area of at least 80% of that of the sizing portion and positioning means for holding the rods in fixed parallel spaced apart relationship in a single plane, the rod positioning means permitting the fabric to be drawn over the rods which are spaced apart by a distance corresponding to the distance between the tubes, driving means for gripping the fabric between the tubes and drawing it over the rods and heating means juxtaposed to the sizing portions of the rods and adjacent to, but spaced from, the inlet portions and cooling means, juxtaposed to the sizing portions of the rods and adjacent to the outlet end of the rods remote from the inlet portions and spaced from the heating means.
The rod positioning means preferably comprises at least two pairs of rollers, each pair of rollers having pairs of opposed grooves in which the rods can rest, one pair of grooves for each rod, the dimensions of the grooves being such as to permit the fabric to be drawn through the grooves over the rods located therein.
The rods preferably have their inlet ends bent so as to form a hook of a walking stick type of shape which engages one of the rollers of the pair of rollers at the inlet end so as to prevent the rods being drawn through the system or causing the system to jam. The inlet ends of the rods are preferably bent through at least 90.degree. but less than 180.degree. and preferably less than 120.degree. and more preferably through 90.degree. only and then extend out away from the roller tangentially thereto.
The ratio of the length of the bent portion to the length of the sizing portion of the rod is desirably in the range 1:3 to 1:10, e.g. 1:5. The ratio of the length of the straight inlet portion to the length of the bent portion is preferably in the ratio 0.1:1 to 0.9:1, e.g. 0.4:1 to 0.7:1.
The driving means are preferably part of the rod positioning means though if desired they may be formed separately. Thus the driving means preferably comprise at least one of a pair of roughened, e.g. knurled, opposed wheel surfaces positioned to grip the fabric between at least one and preferably each pair of rods so that on rotation of the wheel surfaces the fabric will be drawn over the rods without the wheel surfaces cutting the fabric.
The knurled wheel surfaces are preferably provided by at least one, desirably the rear, pair of rod positioning rollers and desirably all the rollers, and are located between each groove of the rollers. The rear pair of rod positioning rollers may be located between the heating means and the cooling means or in the vicinity of the cooling means or more preferably downstream thereof.
The heating means and the cooling means preferably comprise shrouds located close to and on either face of the fabric and supplied with a forced air supply which is heated in the case of the heating means.
It will be appreciated that the invention extends not only to the combination of the machine and the rods but also to the rods on their own and to the machine on its own without the rods.
The rods preferably have a friction reducing and abrasion resistant surface coating at least on their inlet portions. Thus the inlet portions may be metal, e.g. chromium, plated. The floating mandrels may be similarly treated.
We have found that if the fabric is heated before it is brought around the bend of the rods the tubes after they leave the rods tend to bow. We have found that this tendency can be reduced if the material is heated while it goes around the bend and if it is heated on at least part of the straight portion, however, distortion of the fibres is still liable to have occured on going around the bend and joining of the fabric on the rods is still liable to occur.
We have found that these problems can be substantially eliminated by operating in accordance with the invention; thus when the fabric is kept cold while passing around the bend, the resin holds the fibres in position and heating solely on the straight portion produces much better operation of the machine.
When a thermoplastic polymer is used it is desirable to heat the fabric sufficiently to cause the resin to flow, e.g. for a resin of melting point 80.degree. C. a temperature of 110.degree. C. to 120.degree. C. is most satisfactory.
We prefer, however, to apply the drive to the fabric at regions where the fabric is cold or is not being heated, e.g. before the heating stage or after or during the cooling stage.